This invention relates to an improved local area network cabling arrangement. More specifically, it relates to a particular cable design which includes a plurality of metallic conductors which are rotated about their axis prior to being twisted with another conductor to form a conductor-pair. Such insulated conductor rotation (ICR) significantly lowers the pair-capacitance unbalance to ground and structural return loss of the resulting cable by improving the effective concentricity and dielectric eccentricity of the conductors.
Along with the greatly increased use of computers for offices and for manufacturing facilities, there developed a need for a cable which may be used to connect peripheral equipment to mainframe computers and to connect two or more computers into a common network. Of course, given the ever-increasing demands for increased data transmission rates and improved quality, the sought-after cable desirably should not only provide substantially error-free transmission at relatively high bit rates or frequencies but also satisfy numerous other elevated operational performance criteria.
Not surprisingly, of importance to the design of metallic-conductor cables for use in local area networks are the speed and the distances over which data signals must be transmitted. In the past, this need had been one for interconnections operating at data speeds up to 20 kilobits per second and over a distance not exceeding about 150 feet. This need was satisfied with cables which may comprise a plurality of insulated conductors that were connected directly between a computer, for example, and receiving means such as peripheral equipment. Currently, cable, generally identified throughout the industry as Category 3 products, is commercially available that can effectively transmit up to 16 MHz data signals and a series of products designated as Category 5 provide the capability of effectively transmitting up to 100 MHz data signals.
The objectives being demanded by cable customers, including local area network (LAN) vendors and distribution system vendors, are becoming increasingly stringent. This is true for both the breadth of the types of features demanded as well as the technical wherewithal necessary to accomplish the new requests from customers. In this regard, further advances in the operational performance of LAN cables are becoming increasingly difficult.
The unshielded twisted pair has long been used for telephone transmission in the balanced (differential) mode. Over the past several years, some LAN designers, have come to realize the latent transmission capability of unshielded twisted pair cable. Especially noteworthy is the twisted pair""s capability to transmit rugged quantized digital signals as compared to corruptible analog signals.
In an attempt to enhance the operational performance of twisted pair cables, manufacturers have employed a variety of different twist schemes. As used herein, twist scheme is synonymous with what the industry sometimes calls twinning or pairing schemes. In general, twist scheme refers to the exact length and type/lay of twist selected for each conductor pair. More specifically, in one such twist scheme particularly described in commonly-assigned U.S. Pat. No. 4,873,393 issued in the names of Friesen and Nutt and which is hereby expressly incorporated by reference, it is stated that the twist length for each insulated conductor pair should not exceed the product of about forty and the outer diameter of the insulation of one of the conductors of the pair. While this is just one example of an existing approach for defining a twist scheme which results in an enhanced cable design, many others exist.
As a more recent piece of prior art, the reader""s attention is drawn to a unique twist scheme set forth in commonly-assigned patent application filed in the names of Friesen, Hawkins and Zerbs on Jan. 31, 1997 and which is expressly incorporated by reference herein. This document describes a particular series of conductor-pair twist lengths that when used together in a single cable provide operational performance values that significantly surpass the transmission requirements for cables qualifying as Category 5 cables under TIA/EIA-568A. Another commonly-assigned application of interest was filed on Feb. 28, 1997 in the names of Friesen, Hawkins and Zerbs and directed to a cable design that provides significant enhancement in the balance of insertion loss and characteristic impedance from one conductor-pair to other conductor-pairs, and which is also expressly incorporated by reference herein.
Notwithstanding the aforementioned problems and solutions, there still appears to be a need for a cable that satisfies the criteria discussed above and also addresses the need for communication cables, particularly LAN cables, having improved capacitance unbalance and structural return loss characteristics. Notably, capacitance unbalance and structural return loss are becoming increasing targets for improvement as the other operational characteristics are enhanced by ongoing technical development as mentioned above. As a result, the factors that contribute to less than desirable capacitance unbalance and structural return loss values are being evaluated. In particular, the present invention focuses on concentricity and eccentricity of the metallic conductor and its dielectric insulation to develop a technique for compensating for any such problems that may exist in order to improve the capacitance unbalance and structural return loss values of the resulting cable.
The foregoing problems have been overcome by a cabling arrangement of this invention which includes a first conductor-pair including two insulated metallic conductors twisted together with a predetermined first twist length, and at least one additional conductor-pair including two insulated metallic conductors twisted together with a predetermined second twist length different than the first twist length of the first conductor-pair. Most notably, in accordance with the present invention, at least one of the individual conductors has been rotated about its central axis at a predetermined rate of revolution prior to being combined with another conductor to establish a conductor-pair. This insulated conductor rotation (ICR) significantly improves the capacitance unbalance and structural return loss characteristics of the resulting cable.
In an alternative embodiment, the present invention is directed to a communication cable comprising a plurality of insulated conductors surrounded by a sheath system that includes a plastic jacket said conductors being formed into one or more separate units by twisting pairs of the conductors together at a predetermined twist rate, wherein at least one of the separate units comprises at least one conductor that individually rotates about its central axis at a predetermined rotation rate.